1. Field of the Invention
The present invention relates to multi-purpose impact sensors suitable for use in burglar-alarm systems for automobiles or in security systems for access control areas.
2. Description of the Prior Art
As well known to those skilled in the art, burglar-alarm systems for automobiles or security systems for access control areas are conventionally provided with impact sensors, such as piezo-electric sensors or magnetic-coil sensors.
A conventional piezo-electric impact sensor is operated as follows. That is, when external impact is applied to the piezo vibration plate of a piezo-electric impact sensor, the capacitance of the vibration plate is changed. In such a case, the sensor amplifies the capacitance signal and senses the external impact. However, such a piezo-electric impact sensor is problematic in that it uses a vibration plate made of corrosion liable metals. Due to such a corrosion liable vibration plate, the piezo-electric sensor is shortened in the expected life span, and so it is necessary to frequently change the existing sensor of a system with a new one. In addition, the sensitivity of the piezo-electric sensors is easily reduced due to such corrosion liable vibration plates. This finally reduces the operational reliability of the systems.
On the other hand, a conventional magnetic-coil sensor is operated as follows. That is, when external impact is applied to a magnet hung on a flexible suspending means, for example, made of rubber, the magnet is vibrated. In such a case, an induced voltage is generated by a coil positioned around the magnet prior to being amplified, thus allowing the sensor to sense the external impact.
However, it is necessary for such a magnetic-coil sensor to be provided with a means for suspending the magnet. In addition, a coil, having a magnetic core, has to be arranged around the suspended magnet. Therefore, the magnetic-coil sensor is problematic in that the process of assembling the sensor is complicated. In addition, the size of the magnetic-coil sensor is undesirably larger than the other elements of a system due to both the suspending means and the coil.
In the prior art, ultrasonic sensors or infrared sensors, designed to sense a movement of an intruder, may be used in the burglar-alarm systems for automobiles or in the security systems for access control areas. However, it is necessary for these types of sensors to emit powerful ultrasonic rays or infrared rays, and so the sensors exceedingly consume electric power. Another problem of the ultrasonic or infrared sensors resides in that they are expensive and are undesirably limited in sensing area. In addition, such ultrasonic or infrared sensors are designed to sense any movement without discrimination, thus frequently raising unnecessary alarms. This limits use of the ultrasonic or infrared sensors.
In an effort to overcome such problems, the inventor of this invention proposed a multi-purpose impact sensor in Korean Patent Application No. 1994-19393. The above multi-purpose impact sensor is designed to sense an air vibration within a sealed space using a condenser microphone, with the air vibration being caused by external impact applied to the sensor.
In the above multi-purpose impact sensor, a metal weight is installed at the top portion within a sealed elastic rubber cap while being movable in all directions, with a condenser microphone being installed at a position under the weight. When external impact is applied to the sensor, the metal weight swings while changing the pressure within the sealed rubber cap. The changed pressure of air or the air vibration within the sealed cap is transmitted to the condenser microphone, thus allowing the microphone to generate an electric signal.
The above-mentioned multi-purpose impact sensor is free from corrosion different from the conventional piezo-electric sensors. Another advantage of the multi-purpose impact sensor resides in that it accomplishes a desired sensitivity and has a small volume while being produced at low cost different from the conventional magnet-coil sensors. The multi-purpose impact sensor is designed to conserve electric power and to be usable for various purposes different from the conventional ultrasonic or infrared sensors, thus being preferably and widely used for multi-purpose.
However, the above multi-purpose impact sensor is problematic in that it is necessary to form a sealed rubber cap for allowing the vibration plate of the condenser microphone to be vibrated by external impact. In order to prevent the sensor from malfunctioning by unexpected air currents or noises other than external impact, a protection cap has to be installed outside the sealed rubber cap. During the process of producing the multi-purpose impact sensor, the rubber cap has to be hermetically sealed using a special adhesive. This reduces productivity and increases the production cost while producing such impact sensors.
In the above multi-purpose impact sensor, it is necessary to use a spring, which holds the weight while allowing the weight to swing in response to external impact. Such a spring enlarges the volume of the sensor and complicates the process of producing the sensors. It is also very difficult to precisely set the elastic modulus of the spring during a heat treatment process for the spring. This finally allows the sensitivity of the sensor to be nonuniform.